1. Field of the Invention
The present invention relates to the field of injection-moulding technology. It concerns an injection-moulding tool for the production of mouldings in disc form, in particular in the form of information carriers such as CDs and/or DVDs,
2. Description of the Related Art
For producing optically readable information carriers such as audio CDs, CD-ROMs, video CDs or DVDs, injection-moulding tools which form an injection mould by means of two (cylindrical) mirror blocks lying opposite each other and movable in relation to each other are used. On one of the two mirror blocks there is usually releasably fastened a thin, disc-shaped stamper, which stamps the tracks containing the information during the injection moulding of the information carrier being formed. At the outer edge, the injection mould is often bounded by a form ring, which concentrically surrounds one of the mirror blocks.
The form ring projects beyond the planar moulding surface of the mirror block enclosed by it and is displaceable in relation to this mirror block in the axial direction. If the injection mould is closed by an axial movement of the two mirror blocks towards each other, the form ring comes to rest on the opposite mirror block on account of the projection and closes the mould, while the mirror block concentrically enclosed by it is at a predetermined distance from the opposite mirror block. The hot plastic is then injected under high pressure into the disc-shaped cavity formed in this way. Once the cavity of the injection mould has been filled, if need be the mirror block enclosed by the form ring is moved a little towards the other mirror block—in order to achieve a high quality of the information carrier—, while the form ring remains fixed in place on the opposite mirror block on account of the stop (see in this respect FIGS. 2 and 3 of WO-A1-99/37471.
In the case of such moulds, the temperature control of the mirror blocks can take place actively, it being possible by this temperature control on the one hand to bring the mirror blocks to a specific increased temperature in preparation (heating up), if need be only within certain phases of the production process, and it being possible in particular on the other hand to carry away the heat introduced into the mould by the hot injected plastic (cooling).
The possibilities of such temperature control are described for example in EP-A-0899075, and also in EP-A-0864411. In this case, the typical procedure is that channels in which an appropriate medium is carried are provided in the mass of the mirror blocks. Such channels are in this case made to follow a spiral path in the mirror block. In order to allow different capacities in different radial regions of the mirror blocks, it is possible to activate different regions of the spirals differently by means of supplying cooling medium differently.
One of the problems of such solutions is that normally, because of the required high homogeneity of the temperature distribution on the mirror block, a multiplicity of channels that are thin and arranged comparatively close together is demanded, and that correspondingly long channels result. This leads to a cooling response time that is too long for present-day cycle times.